1) Field of the Invention
The present invention relates to an observation optical system. To be specific, it relates to an observation optical system used in an image display apparatus that can be held on the head or face of an observer and that can be attached to a cellular phone or a portable intelligent terminal.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a wide-screen image. Also, in accordance with recent popularization of cellular phone and portable intelligent terminal, requirements for large view of graphics or text data on these apparatuses have grown.
Conventionally, as head-mount image display apparatuses, there are known a type in which an image on an image display element such as a CRT is transmitted via an image transmitting element to an object surface, where the image is projected in the air by a toric reflecting surface (U.S. Pat. No. 4,026,641), and a type in which an image displayed on a liquid crystal display device (LCD) is once imaged in the air via a refraction-type relay optical system and then is introduced into an eye of an observer via an eyepiece optical system constructed of a concave mirror arranged in a decentered manner (Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 6-294943).
However, these types of the head-mount image display apparatus are not suitable for application to a cellular phone or portable intelligent terminal because the observation optical system, specifically the relay optical system arranged on the side of the image display element, is too large.
Also, if see-through function is to be provided, it is necessary to construct an exit pupil-side combiner surface with a half mirror, which causes loss of light for an image from an image display element and for a see-through image.
Therefore, an object of the present invention is to provide an observation optical system which is made so compact as to be applicable, as an image display apparatus, to a cellular phone or a portable intelligent terminal, and which provides an electronic image and a see-through image with sufficient brightness while achieving high image definition.
An image observation optical system according to the present invention comprises a first unit with a positive refracting power and a second unit, wherein the first unit comprises at least one prism member having a positive refracting power, and the second unit is composed of a diffraction element having a lens function caused by diffraction.
Also, it is desirable that, in the observation optical system, the first unit has an action of imaging an electronic image for obtaining a relay image and the second unit has an action of forming an exit pupil for introducing the relayed image to an observer, wherein the prism member of the first unit comprises at least one decentered prism with decentered surfaces, which has an entrance surface through which bundles of rays emergent from the image display element enter the prism, at least one reflecting surface which reflects the bundles of rays inside the prism, and an exit surface through which the bundles of rays exit out of the prism, at least one of the exit surface and the reflecting surface is shaped as a curved surface which exerts a power on the bundles of rays, and the curved surface is a rotationally asymmetric surface constructed and arranged to compensate aberrations generated by decentering.
Also, according to the present invention, the diffraction element used in the second unit is a reflection-type one.
Also, under the condition where a reflecting surface having a power is decentered from the axial chief ray, which is defined as a ray travelling from the center of the object point via the center of the pupil through the center of the image surface, as traced in the reverse direction, it is desirable that at least one of surfaces included in the prism used in the present invention is shaped as a rotationally asymmetric surface. It is particularly preferred that at least one reflecting surface of the prism member is shaped as a rotationally asymmetric surface in view of compensation of aberrations.
In this case, the reflecting surface formed on the prism member may be configured as a plane-symmetric free curved surface defining only one plane of symmetry.
It is also desirable that the surface of the diffraction element used in the present invention is shaped as a rotationally asymmetric surface. The base surface on which the diffraction element is provided may be shaped as any one of a cylindrical surface, a spherical surface, an aspherical surface, an anamorphic surface, a toric surface, a surface that defines only one plane of symmetry, and a plane-symmetric free curved surface.
Also, a rotationally asymmetric surface used in the present invention may be configured as any one of an anamorphic surface, a toric surface, and a free curved surface that defines only one plane of symmetry. Specifically, the surface is preferably configured as a free curved surface that defines only one plane of symmetry.
Also, according to the present invention, it is desirable that the second unit is constructed to have low diffraction efficiency for rays transmitted therethrough so as not to exert optical power on them.
Also, according to the present invention, it is desirable that the first unit is provided with a diffraction element.
Also, in the second unit, it is desirable to use a volume hologram having a rotationally asymmetric power.
In this case, it is desirable that the first unit is provided with a volume hologram surface having a rotationally asymmetric power.
Also, according to the present invention, it is desirable that the diffraction element of the first unit is a transmission-type volume hologram.
Also, it is desirable that the diffraction element of the first unit is formed on the exit surface of the prism member.
Also, according to the present invention, it is preferred that the diffraction element of the first unit is provided with a flare stop.
Also, according to the present invention, it is desirable that the diffraction element of the second unit is a reflection-type volume hologram.
Also, according to the present invention, it is preferred that the diffraction element of the second unit acts as a flare stop also.
Also, according to the present invention, it is preferred that the prism member has at least two reflecting surfaces constructed and arranged to reflect bundles of rays inside the prism, wherein the two reflecting surfaces are shaped as curved surfaces to exert an optical power on the bundles of rays and the curved surfaces are rotationally asymmetric surfaces constructed and arranged to compensate aberrations generated by decentering.
Also, according to the present invention, it is preferred that the prism member is configured so that the direction of the axial chief ray as emergent from the prism member is averted from the exit pupil.
In this case, it is preferred the direction of the axial chief ray as emergent from the prism member satisfies the following condition (1):
xe2x88x9225xc2x0xe2x89xa6xcex8xe2x89xa625xc2x0xe2x80x83xe2x80x83(1) 
where xcex8 is a tilt angle of the axial chief ray in reference to X direction, which is taken as 0xc2x0, upon X direction being defined as a direction lying in a plane of decentering of each surface constituting the prism member of the first unit and perpendicularly intersecting Z direction, which is defined by the straight line portion of the axial chief ray from the center of the exit pupil to the entrance surface of the second unit.
In this case, it is much preferred that the following condition (2) is satisfied:
xe2x88x9215xc2x0xe2x89xa6xcex8xe2x89xa615xc2x0xe2x80x83xe2x80x83(2) 
Further, it is still much preferred that the following condition (3) is satisfied:
xe2x88x925xc2x0xe2x89xa6xcex8xe2x89xa65xc2x0xe2x80x83xe2x80x83(3) 
Also, according to the present invention, it is desirable to cover the first unit with a dust shield member.
In this case, the dust shield member of the first unit is preferably provided with an encasement which accommodates the first unit, and a transparent cover arranged on the exit side of the first unit so that light is introduced into the second unit through the cover.
In this configuration, upon the transparent cover being made of transparent material such as glass or plastic, the diffraction element of the first unit is preferably provided on a surface of the transparent cover, using it as a base, from inside the encasement.
Also, an image display element, a main frame in which any one of the above-mentioned observation optical system of the present invention is arranged as an eyepiece optical system, and a support member which is constructed to be mounted on the lateral sides of the head of an observer so as to hold the main frame in front of the face of the observer can be combined into a head-mount type image display apparatus.
In this case, the head-mount type image display apparatus may be configured so that the observation optical system and spectacle lenses are integrally arranged in the main frame.
Alternatively, the head-mount type image display apparatus may be configured so that the support member is constructed to achieve removable mount to side frames of spectacles.
Also, a pair of the observation optical systems can be arranged in parallel as left and right systems so as to configure a head-mount type binocular image display apparatus.
This and other objects as well as features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.